Advertisers try to reach consumers in a variety of ways. One of those ways is through web-based advertising on the Internet. A common way for advertisers to reach consumers on the Internet is by presenting ads in web sites, either as a pop-up window or embedded within the web page being viewed by the consumer. Web sites, such as MSN, MySpace, and Yahoo, rely on advertising as an increasingly important source of revenue. But unless advertisers realize returns from advertising on a web site (e.g., in the form of a purchase or a visit to the advertiser's web site), advertisers may reduce or even stop advertising on the web site. Accordingly, web sites have an interest in selecting an ad that generates revenue for the web site and provides some form of return to the advertiser.